The origin of this rifle can be traced back to the final years of World War II when Mauser engineers at the Light Weapon Development Group (Abteilung 37) at Oberndorf am Neckar designed the MKb Gerät 06 (Maschinenkarabiner Gerät 06 or "machine carbine device 06") prototype assault rifle chambered for the intermediate 7.92×33mm Kurz cartridge, first with the Gerät 06 model using a roller-locked short recoil mechanism originally adapted from the MG 42 machine gun but with a fixed barrel and conventional gas-actuated piston rod.[3] It was realized that with careful attention to the mechanical ratios, the gas system could be omitted.[4] The resultant weapon, the Gerät 06H (the "H" suffix is an abbreviation for halbverriegelt or "half-locked") was assigned the designation StG 45(M) (Sturmgewehr 45(M) or assault rifle) but was not produced in any significant numbers and the war ended before the first production rifles were completed.[5]

The German technicians involved in developing the StG 45(M) were taken to work in France at CEAM (Centre d'Etudes et d'Armement de Mulhouse). The StG 45(M) mechanism was modified by Ludwig Vorgrimler and Theodor Löffler at the Mulhouse facility between 1946 and 1949. Three versions were made, chambered in .30 Carbine, 7.92×33 mm Kurz as well as the experimental 7.65×35 mm French short cartridge developed by Cartoucherie de Valence in 1948. A 7.5×38 mm cartridge using a partial aluminium bullet was abandoned in 1947. Löffler's design, designated the Carabine Mitrailleuse Modèle 1950, was retained for trials among 12 different prototypes designed by CEAM, MAC, and MAS. Engaged in the Indochina War and being the second[clarification needed]NATO contributor, France canceled the adoption of these new weapons for financial reasons.

In 1950, Vorgrimler moved to Spain where he created the LV-50 rifle chambered for the Kurz cartridge and later, the proprietary 7.92×40 mm CETME M53 round.[6] At this point, the rifle was renamed the Modelo 2. The Modelo 2 drew the attention of the West GermanBorder Guards (Bundesgrenzschutz), who sought to re-equip the newly formed national defense forces. Not willing to accept a cartridge outside of the NATO specification, the Germans asked CETME to develop a 7.62×51 mm version of the rifle. The resulting CETME Model A was chambered for the 7.62×51 mm CETME cartridge which was identical in chamber dimensions but had a reduced-power load compared to the 7.62 mm NATO round. Further development of the rifle with input from H&K produced the CETME Model B which received several modifications, including the ability to fire from a closed bolt in both semi-automatic and automatic firing modes, a new perforated sheet metal handguard (the folding bipod had been the foregrip in previous models), improved ergonomics and a slightly longer barrel with a 22 mmrifle grenade launcher guide. In 1958, this rifle was accepted into service with the Spanish Army as the Modelo 58, using the 7.62×51 mm CETME round.

In 1956, the Bundesgrenzschutz canceled their planned procurement of the CETME rifles, adopting the Belgian-made FN FAL (G1) instead. However, the newly formed West German Army (Bundeswehr) now showed interest and soon purchased a number of CETME rifles (7.62×51 mm NATO chambering) for further testing. The CETME, known as the Automatisches Gewehr G3 according to German nomenclature, competed successfully against the SwissSIG SG 510 (G2) and the AmericanAR-10 (G4) to replace the previously favored G1 rifle. In January 1959, the Bundeswehr officially adopted the CETME proposal. The West German government wanted the G3 rifle to be produced under license in Germany; purchase of the G1 had previously fallen through over FN's refusal to grant such a license. In the case of the G3, the Dutch firm Nederlandse Wapen en Munitiefabriek (NWM) held production and sales rights to the CETME design outside of Spain. To acquire production rights, the West German government offered NWM contracts to supply the German Air Force (Luftwaffe) with 2 0mm ammunition. Production of the G3 was then assigned to Rheinmetall and H&K. The latter company already had ties to CETME, and had worked to further optimize the CETME rifle for use with the full-power 7.62×51 mm NATO cartridge (as opposed to the downloaded CETME variant). In 1969, Rheinmetall gave up production rights to the G3 in exchange for H&K's promise not to bid on MG 3 production. Later in 1977, the West German government ceded ownership of G3 production and sales rights exclusively to H&K.

The G3A3 (A4) is a selective-fire automatic weapon that employs a roller-delayed blowback operating system. The two-piece bolt assembly consists of a breech (bolt head) and bolt carrier. The bolt is held in battery by two sliding cylindrical rollers that engage locking recesses in the barrel extension (popularly called a "trunnion"; BATF calls this a "mounting block"). The breech is opened when both rollers are compressed inward against camming surfaces driven by the rearward pressure of the expanding gases upon the bolt head. As the rollers move inward, recoil energy is transferred to the locking piece and bolt carrier which begin to withdraw while the bolt head slowly moves rearward in relation to the bolt carrier. As the bolt carrier clears the rollers, pressure in the bore drops to a safe level, the bolt head is caught by the bolt carrier and moves to the rear as one unit, continuing the operating cycle. The bolt also features an anti-bounce mechanism that prevents the bolt from bouncing off the barrel's breech surface. The spring-powered claw extractor is also contained inside the bolt while the lever ejector is located inside the trigger housing (actuated by the recoiling bolt).[6]

The rifle is hammer fired and has a trigger mechanism with a 3-position fire selector switch that is also the manual safety toggle that secures the weapon from accidentally discharging (fire selector in the "E" or "1" position – single fire mode ("Einzelfeuer"), "F" or "20" – automatic fire ("Feuerstoß"), "S" or "0" – weapon is safe ("Sicher"), trigger disabled mechanically). The weapon can be fitted with an optional 4-position safety/fire selector group illustrated with pictograms with an ambidextrous selector lever. The additional, fourth selector setting enables a 3-round burst mode of fire.[6]

The firearm is equipped with iron sights that consist of a rotary rear drum and hooded front post. The rear sight, mechanically adjustable for both windage and elevation, has an open notch used to fire up to 100 m and three apertures used for: 200, 300 and 400 m.[6] The receiver housing has recesses that work with HK clamp adapters used to mount day or night optics.

The rifled barrel (contains 4 right-hand grooves with a 305 mm twist rate) terminates with a slotted flash suppressor which can also be used to attach a bayonet or serve as an adapter for launching rifle grenades. From the G3A3 the barrel had polygonal rifling.[7] The barrel chamber is fluted, which assists in the initial extraction of a spent cartridge casing (since the breech is opened under very high barrel pressure).[6]

The G3A3 (A4) uses either steel (260 g) or aluminium (140 g) double-stacked straight box magazines, or a 50-round drum magazine. H&K developed a prototype plastic disposable magazine in the early 1960s, but it was not adopted as aluminum magazines were just as light and proved more durable, as well as easier to produce.

Standard accessories supplied with the rifle include: a detachable bipod (not included with rifles that have a perforated plastic handguard), sling, cleaning kit and a speed-loading device. Several types of bayonet are available for the G3, but with few exceptions they require an adapter to be inserted into the end of the cocking tube. The most common type features a 63⁄4 inch spear-point blade nearly identical with the M7 bayonet, but with a different grip because of its mounting above the barrel. The weapon can also mount a 40 mm HK79 under-barrel grenade launcher, blank firing adapter a straight blowback bolt (called a "PT" bolt, lacks rollers) used for firing 7.62×51 mm ammunition with plastic bullets, a conversion kit used for training with .22 Long Rifle ammunition and a sound suppressor (that uses standard ammunition).

The G3 is a modular weapon system. Its butt-stock, fore-stock and pistol-grip/fire-control assembly may be changed at will in a variety of configurations (listed below). Simple push-pins hold the components in place and removing them will allow the user to remove and replace parts rapidly.

Apart from the G3A3 and G3A4 HK also built: the G3A3ZF (essentially a G3A3 with a Hensoldt 4×24 optical sight), the accurized G3SG/1 rifle (hand-selected G3A3s, equipped with an improved trigger, Zeiss telescopic sight with a variable 1.5–6× magnification and a cheek riser) and the G3K carbine which uses an HK33 handguard and a short barrel (reduced in length to the base of the front sight post), that is too short for use with a bayonet or rifle grenades.[6]

The G3 served as a basis for many other weapons, among them: the PSG1 and MSG90 precision rifles, the HK11 and HK21 family of light machine guns, a semi-automatic version known as the HK91, a "sporterized" model called the SR9 (designed for the civilian market in countries where the HK91 would not qualify, primarily the US after the 1989 importation restrictions) and the MC51 carbine.

G3: Original model based on the CETME Modelo B. It had a wooden stock and handguard.

G3A1: G3 with a single-position, collapsible stock. This design was chosen after earlier experimentation with an MP-40 style ventrally-folding metal stock; excessive recoil caused it to be dropped from consideration.

G3A2: G3 with new rotating drum rear sight.

G3A3: The most well known version. Drum sights, a fixed plastic buttstock, and a plastic handguard that does not contact the barrel. The handguard came in a slim, ventilated version and a wide version. The latter allows for the attachment of a bipod.

G3A3A1: This is a version of the G3A3 with an ambidextrous trigger group and brass deflector. This is an official German Army designation, not an HK factory one.

G3A4: The G3A4 uses drum sights and a single position, collapsible stock.

G3A4A1: This is a variant of the G3A4 with an ambidextrous trigger group and brass deflector. This is an official German Army designation, not an HK factory one.

G3KA4: Smallest of the line, it is a Karabiner, or carbine version of the G3. It features drum sights, a retractable stock, and a 315 mm (12.4 in) barrel.

G3KA4A1: Variant of the G3KA4 with an ambidextrous trigger group and brass deflector. This is an official German Army designation, not an HK factory one.

G3A5: HK assigned model number for the HK-made Danish version of the G3A3. It differs in that it has a silent bolt-closure device. In Danish service it is known as the Gv M/66. The Gv M/66 was originally intended for use with optics as a designated marksman rifle, while the rest of the squad were issued M1 Garands.

G3A6: HK assigned model number for the Iranian-made version of the G3A3. It differs in having a dark-green handguard, stock, and trigger pack.

G3A7: HK assigned model number for the Turkish-made version of the G3A3.

G3A7A1: HK assigned model number for the Turkish-made version of the G3A4.

HSG1: HK assigned model number for the Luxembourg-made version of the G3A3.

Gv M/75: Variant leased from the German Bundeswehr / German government by the Danish government to replace the aging M1 Garands. Originally manufactured by either Rheinmetall or HK for the German Bundeswehr. The Gv M/75 rifles are basically G3's with the old style straight cocking tube as opposed to the later FS (Freischwinger, "Cantilevered") variant. The Rheinmetall versions lack an external selector lever and can be converted from semi-automatic to full automatic (or vice versa) by the use of a special tool.

NDR/StG-59: The NDR assault rifle was an entrant into the East German armed forces. It externally resembled the G3 but internally the Sturmgewehr 44. It was chambered in the 7.62x39mm round fed from 30 round magazines and gas operated using a tilting bolt. It was resistant to the likes of dirt but found too complicated when field stripping. The NDR was trialled against the Czechoslovak Vz58. The project was cancelled as the AK-47 was preferred.[8]

AG-3: Norwegian G3A5 variant produced by Kongsberg Våpenfabrikk. A total of 253,497 units were produced for the Norwegian Armed Forces from 1967 to 1974. The Norwegian AG-3 differs from the original G3; it has a buttstock that is approx. 2 cm longer, the bolt carrier has a serrated thumb groove to aid in silent bolt closure, it features an all-metal cocking handle and a different bayonet mount. On April 11, 2007, it was announced that the AG-3 would be replaced by the Heckler & Koch HK416, in all military branches except for certain groups of the Home Guard.

AG-3F1: An AG-3 with a retractable stock as on G3A4. Produced by Kongsberg Våpenfabrikk. A retractable stock was required by certain groups of soldiers within the Norwegian Armed Forces, primarily vehicle crews with limited space inside, particularly where a quick disembarkment from such a vehicle is required. All versions of the AG-3 have the ability to attach a 40 mm HK79 grenade launcher.

AG-3F2: An improvement of the AG-3F1, featuring Picatinny rails on the receiver as well as the foregrip.

Ak 4: Swedish-made version of the G3A3, with a buttstock that is approx. 2 cm longer, the bolt carrier has a serrated thumb groove to aid in silent bolt closure and fitted with a heavy buffer for higher number of rounds fired before failure. The rifles were manufactured from 1965 to 1970 by both Carl Gustafs Stads Gevärsfabrik and Husqvarna Vapenfabrik and from 1970 until the end of production in 1985 – exclusively by Gevärsfabrik in Eskilstuna. All Ak 4s are adapted to mount the M203 grenade launcher. Sweden has supplied unmodified Ak4s to Estonia, Latvia and Lithuania.

Ak 4OR: Optiskt Riktmedel, optical sight. This model is fitted with a Hensoldt 4×24 telescopic sight mounted via a HK claw mount. During a few years it was not issued but it is now again in use by the Hemvärnet - Nationella skyddsstyrkorna ("Swedish Home Guard").

Ak 4B: In this updated version the iron sights have been removed and replaced with an Aimpoint CS red-dot reflex sight mounted on a Picatinny rail. The rail is welded onto the rifle. Used by Hemvärnet-Nationella skyddsstyrkorna ("Swedish Home Guard").

G3TGS: This is simply a G3 with a 40 mm HK79 under-barrel grenade launcher. TGS stands for Tragbares Granat System ("portable grenade system").

G3A3ZF: This is a rifle issued with a claw-and rail scope bracket and a 4 x 24 mm Hensoldt scope. The ZF stands for Zielfernrohr or "telescopic sight".

G3SG/1: An accurized variant of the G3. The "SG" stands for Scharfschützengewehr, or "sharpshooting rifle". The rifles were individually selected from the production line for their accuracy and then modified. A Zeiss 1.5-6x variable power telescopic sight was added using a HK claw mount attached to the receiver. The stock was extended slightly compared to the standard G3 fixed stock and comes with a heavy, dual-stage buffer and adjustable cheek rest. A special set trigger group was added for a crisp trigger pull of 1 lb. Automatic fire was retained.

MSG3: A variant featuring the newer scope mount that is found on only a few of the Heckler & Koch rifles, as compared to the more conventional claw mounts, though the claw mounting points remain on the receiver. This newer scope mount does not allow the use of the open sights with the mount in place, as is the case with the more conventional claw mount.

PSG-1: A free-floating barrel semi-auto only version of the G3 with numerous other upgrades and such to meet the necessities of police sniper units. This rifle is famous for its accuracy and comfort, but infamous for its price and inability to be deployed because some upgrades made the rifle too fragile.

MSG-90: A somewhat cheaper version of the PSG-1 modified for military applications.

HK 32: An experimental variant of the rifle chambered for the Soviet 7.62x39mm M43 cartridge. It was never adopted by any country.[9]

G3A1*: The terminology used by custom gunsmiths (e.g., Choate) and importers (e.g., Interarms) for imported semi-automatic G3 weapons with an aftermarket side-folding stock. This is not part of official HK nomenclature.

HK41: The HK41 is a semi-automatic version of the G3 that was marketed to law-enforcement. Limited sales at home and US import restrictions and firearms regulations led this weapon to be dropped from HK's product line quickly and it was replaced by the HK91.

Fleming Arms HK51: Contrary to popular belief the HK 51 is not made by H&K, being a creation of the American Class II manufacturing after-market. The HK 51 has no real standards but is usually a cut down and modified G3A3 or its semi-automatic clones the HK 41 and HK 91 and modified to take MP-5 furniture and accessories. It is usually fitted with a collapsible stock; with a 211 mm (8.31-inch) long barrel; it is a tiny 589 mm (23.17 inches) with the stock retracted and 780 mm (30.72 inches) with the stock extended. The first commercial version was by Bill Fleming of Fleming Arms and existed before Heckler & Koch made the HK53.

HK91: The HK91 is a semi-automatic version of the G3 similar to the HK41, also marketed to civilians. However, in order to comply with US firearm regulations a number of modifications to the HK91 were made that do not appear on the first pattern HK41. Internal parts that could allow fully automatic fire were removed. A shelf was welded onto the receiver where the push-pin of the trigger pack would normally go, to prevent installation of a fully automatic trigger pack. This did not allow the use of the paddle style magazine release and so the magazine release button on the right side of the magazine well must be used instead. It is otherwise identical to the G3A3/A4. Importation into the United States began in 1974 and ceased in 1989, with some 48,000 rifles being imported.

HK911: The HK911 was an HK91A2 with the flash hider removed and the receiver re-stamped with an extra 1 to comply with the US importation ban of 1989. The new designation theoretically made it legally immune to the Import Ban, as no "HK911" rifles were mentioned on the list of banned guns. However, the later banning of several "paramilitary" features on the HK911 made it illegal.

SR9: These variants of the HK91ZF were created to comply with the Semi-Auto Import Ban of 1989, which included all variants of the HK91. They differed from the HK91 in that they had their flash hiders removed and featured a smooth forend that lacked the bipod attachment point. The SR9 series were banned from importation to the United States by President Clinton in 1997 because they could accept standard-capacity magazines. The SR9 was an HK91A2ZF with the pistol grip and buttstock replaced with a one-piece thumbhole stock.

SR9 (T): The (T), or "Target", model was an HK91A2ZF with the trigger replaced with the PSG-1 model, the pistol grip replaced with an ergonomic PSG-1 grip and the buttstock replaced with an MSG90 model.

SR9 (TC): The (TC), or "Target Competition", model was an HK91A2ZF with the trigger group, pistol grip and buttstock derived from the PSG-1.

PTR 91 Series: Additionally, JLD started manufacturing semi-automatic copies of the HK G3 called the PTR 91 in the United States. They used tooling from the FMP arms factory in Portugal to build the rifles.

SAR-3: Semi-automatic copy of HK-91 made by EBO in Greece and imported into the United States by Springfield Armory.

SAR-8: Post-ban version of SAR-3, modified to comply with import restrictions.

Denmark: G3A5, as the Gevær Model 1966 (Gv M/66). Another variant, designated Gevær Model 1975 (Gv M/75) was leased from the German government.[14] All G3s in Danish service are being replaced by the Diemaco C7 (Gv m/95), and Diemaco C8 (Gv m/96).

Germany: Used by the German Army from the 1950s until the mid-1990s as the primary service rifle.[16] Originally to be replaced by the HK G41 and HK G11, post-reunification budget cuts forced the procurement of the HK G36 instead. Large numbers still in storage, and is used in overseas deployments as a designated marksman rifle. Some variants still in use by border guards and police forces.[10]

Nigeria: Produced under license by Defense Industries Corporation.[24]

Norway: AG-3 variant was manufactured locally by Norsk Forsvarsteknologi. Has been replaced by the Heckler & Koch HK416.[25] The AG-3 is still in use by the National Guard "Heimevernet", except the Rapid Response Units (Innsatsstyrkene). Production of the AG3 ended in 1974.

Sweden: Made by 3 manufacturers, Heckler & Koch in Germany, and under license by Husqvarna Vapenfabrik (Years of mfg. 1965–70) and Carl Gustaf Gevärsfaktori (Years of mfg. 1965–80) which was later renamed to Förenade Fabriksverken (FFV) as the Ak 4 (Automatkarbin 4).[32] Two sub-variants are known to exist, one equipped with a rail and Aimpoint sight (Ak4 B) and the other with a 4x magnifying optic, the Hensoldt ZF4×24 (Ak 4OR). It has since been replaced by the Ak 5 (Automatkarbin 5; a modified version of the FN FNC) in the regular army. Ak 4B and Ak 4OR, some times in combination with the M203 grenade launcher, is still in use in Hemvärnet – Nationella skyddsstyrkorna ("Swedish Home Guard").